Signal strength in subsecond FLASH magnetic resonance imaging: the dynamic approach to steady state.
Subsecond fast low-angle shot (FLASH) magnetic resonance imaging (MRI) allows single shot studies of the human heart within measuring times of about 100-300 ms depending on the data matrix. In contrast to conventional FLASH MRI subsecond applications acquire data during the approach to steady state. A detailed analysis of the saturation behavior of the signal is given for the ideal case of a rectangular slice profile. In a second step, realistic slice profiles assuming Gaussian-shaped excitation pulses were taken into account by means of a numerical solution of the Bloch equations. It turns out that the signal strength and the resulting image intensity is considerably higher than may be expected from steady-state considerations. Correspondingly optimized flip angles depend on the number of phase-encoding steps. Assuming long T1 relaxation times as, for example, encountered in muscle and brain tissue and repetition times of 5 ms or less, optimum flip angles are 12 degrees-16 degrees. The use of even higher flip angles (greater than or equal to 20 degrees) causes heavily distorted slice profiles and a dynamic increase of the effective slice thickness. Flip angles of the order of the Ernst angle (6 degrees) correspond to steady-state conditions and lead to considerable signal losses. The theoretical results are confirmed by subsecond FLASH MRI studies of the human heart using a 2.0 T whole-body system (Siemens Magnetom).
Hänicke
W
W
Max-Planck-Institut für Biophysikalische Chemie, Göttingen, Federal Republic of Germany.
Merboldt
K D
KD
Chien
D
D
Gyngell
M L
ML
Bruhn
H
H
Frahm
J
J
D055592:Biophysical Phenomena; D001703:Biophysics; D006321:Heart / Q000033:anatomy & histology*; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging / Q000379:methods / Q000706:statistics & numerical data*; D012815:Signal Processing, Computer-Assisted